A selective catalytic reduction NOx catalyst (which will be sometimes referred to as “SCR catalyst” hereinafter) is known to purify NOx contained in exhaust gas of an internal combustion engine using ammonia as reducing agent. It is known to use an SCR catalyst supported on a filter that traps particulate matter (which will be hereinafter referred to as “PM”) in the exhaust gas. In some cases, an addition valve for supplying urea solution (aqueous solution) to the exhaust gas is provided upstream of the SCR catalyst. Urea solution added to the exhaust gas is hydrolyzed by the heat of the exhaust gas and/or the heat of the SCR catalyst to produce ammonia, which is adsorbed by the SCR catalyst.
Urea solution freezes at, for example, approximately −11° C. When the outside air temperature is low, there is a possibility that urea solution may freeze in the addition valve and/or in a channel for supplying urea solution after the internal combustion engine is stopped. Freezing of urea solution can make it impossible to supply urea solution after the start of the internal combustion engine and/or cause breakage of the addition valve or the channel for supplying urea solution. After the internal combustion engine is stopped, the addition valve receives heat from the exhaust gas and components around the addition valve, so that the temperature of the addition valve temporarily becomes higher than its temperature during the operation of the internal combustion engine. If urea solution remains in the addition valve in such an occasion, precipitates may be produced in the addition valve from the urea solution, which can cause abrasion of sliding parts in the addition valve or clogging of the addition valve. Moreover, the rise in the temperature of the addition valve after the stop of the internal combustion engine sometimes leads to hydrolysis of urea inside the addition valve, producing ammonia. It is possible that ammonia thus produced may cause corrosion of the addition valve.
A conventional countermeasure to the above problem is to suck urea solution remaining in the addition valve and the channel for supplying urea solution back into a tank after the internal combustion engine is stopped (see, for example, PTL 1 in the following list).